There is an increasing demand for continuous or regular monitoring of a patient's vital signs, for assessing cardiovascular function for example. The use of electrocardiograms (ECGs), blood pressure (BP) measurement systems and pulse oximeters is therefore widely known.
Non-invasive measurement of BP using cuff-based methods provides adequate data for many applications in medicine. However, cuff-based methods exhibit disadvantages which can limit their use in certain clinical situations. First, a continuous measurement of blood pressure is not possible, since a pause of at least 1-2 min between two BP measurements is necessary to avoid errors in the measurement. Therefore, short-term changes in BP cannot be detected. Furthermore, the inflation of the cuff may disturb the patient and the consequences of these disturbances are alterations of the BP. Both problems are, for example, important when investigating BP fluctuations during sleep.
A known alternative approach for a continuous, non-invasive and indirect measurement of BP is based on changes in pulse wave velocity (PWV). PWV is the speed of a pressure pulse propagating along the arterial wall. Typically, a relation of blood pressure and PWV in arteries is expressed by the Moens-Korteweg-relation, which can be derived from hydrodynamic theory:
                    c        =                                            hE              t                                      2              ⁢              ρ              ⁢                                                          ⁢              R                                                          (                  Eq          .                                          ⁢          1                )            
Equation 1: Moens-Korteweg-Equation Often Used to Describe the Relation of Pulse-Wave-Velocity and Blood Pressure where: c=pulse wave velocity, Et=tangential elasticity module, ρ=density, R=radius of artery, h=artery wall thickness.
PWV can be calculated from pulse transit time (PTT). PTT is the time a propagating wave takes on the same cardiac cycle between two separate arterial sites. PTT has been shown to be quasi-linear to low BP values, but increases exponentially at higher pressures. Typical known set-ups for measuring PTT include:
1. ECG- and Photoplethysmography (PPG): Wherein PTT is given by the time-difference between R-peak and characteristic points in PPG. The PPG can be measured at various positions on the body e.g. ear or finger;
2. ECG and bio-impedance measurement at arm (impedance plethysmography (IPG)): Wherein PTT is given by the time-difference between an R-peak of an ECG signal and characteristic points in the IPG;
3. Impedance Cardiography (ICG) of the thorax and bio-impedance measurement at arm (IPG): Wherein PTT is given by the time-difference between characteristic points in the ICG and characteristic points in the IPG; and
4. Impedance plethysmogram (IPG1) at a first position on an arm and bio-impedance measurement at a second position on an arm (IPG2): Wherein PTT is given by the time-difference between characteristic points in the IPG1 and characteristic points in the IPG2.
However, when clinical standard sensors or methodologies are used, the above methods for measuring PTT have several disadvantages, especially for personal healthcare applications. State of the art sensors, such as finger or ear sensors, measuring a photoplethysmogram or bio-impedance methodologies are an inconvenience in normal life requiring PPG sensors or special medical electrodes that must be glued to the skin. Accordingly, such methodologies for measuring PTT are not suitable for long term continuous or regular monitoring of a patient's BP.